All enlarge and reach their final shapes and sizes

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Unformatted text preview: of age, every- Where Our Sexes Come From 55 thing shuts down for a bit. This is called the “juvenile pause,” when the hypothalamus, pituitary, and gonads put their feet up and take a well-deserved break.2 The hormone storm abates, bones grow, muscles strengthen, brains wire and rewire themselves. But the sexual cyclone is still on course, and in most of us it will make landfall by about age twelve. Shortly before the final stage of puberty, neurons within our hypothalamuses rise from their slumber and, while the rest of our bodies sleep, again begin to pump GNRH hormone into our pituitaries. Our pituitaries respond by releasing follicle-stimulating and luteinizing hormones. Then, every night during sleep, about every ninety minutes our gonads get a jolt of hormones. Ovaries begin to release estrogen, testes create testosterone and dihydrotestosterone, and the shapes of our bodies begin to change. Swelling of one or both areolae (the circle around the nipples) and the first appearance of pubic hair along the labia mark the first brushstrokes of puberty in girls. Six to twelve months later that swelling has spread to both breasts and reaches beyond the areolae. At the same time, the pubic hair spreads to the pubic mound. About a year later, breasts have enlarged to nearly final size, and pubic hair has assumed the adult triangle-shaped appearance. Internally, another map is unfolding. The vagina, uterus, and ovaries all respond to this new spate of hormones as well. All enlarge and reach their final shapes and sizes. And, most of the time, ovulation and menstruation begin. In infants with one X and one Y chromosome, usually the penis that formed before birth enlarges during the first four years of infancy, and then rests. At about age eleven, the testes begin to produce testosterone, the penis and scrotum enlarge, and pubic hair begins to appear. Ejaculation becomes possible early in puberty though ejaculates may contain relatively few sperm. At about the same time, the penis enlarges, and pubic hair becomes more abundant. About twelve months later the adult, triangle-shaped growth of pubic hair appears. And in the last stages, pubic hair spreads to the thighs and abdomen, facial hair begins to grow, and voices deepen. Most of the time. 56 Between XX and XY And most of the time all of these things happen because of those once-every-ninety-minute pulses of hormones put out by our hypothalamuses—small pistons compressing and relaxing, driven by their own biorhythms, pushing us, while we sleep, into adulthood. In response to these periodic squirts of GNRH, our pituitaries begin a cycle of on-off production of luteinizing and follicle-stimulating hormones. These hormones, in turn, seek out our testes or our ovaries and push them to secrete the all-important sex steroid hormones, estradiol and testosterone. In men, at the same time, another form of the enzyme 5-alpha reductase type 1 appears. This enzyme, much like the one produced during fetal life, converts testosterone into 5-alpha dihydrotestosterone, another key needed to open the last gates on the way to male development. Again, the entire orchestra must contribute. If even a single instrument fails to deliver its part on time and in proper syncopation, the whole symphony may crash. Hormones, enzymes, genes, neurons, receptors, and signal transduction machinery can shatter into a million shards, and the images those shards reflect can be as varied as our imaginations. And as complex as all that may seem, it’s only part of the story. The adrenal glands are another set of critical organs that form with the genital ridge as it develops in fetuses. The adrenal glands also play critical roles in the process of human sexual development. In response to hormones produced by the hypothalamus and the pituitary glands during fetal development and during puberty, the adrenal glands typically make and secrete several additional hormones. Among these are the adrenal androgens—dehydroepiandrosterone (DHEA), androstenedione, and pregnenolone. These compounds by themselves don’t do very much. But in the blood, enzymes convert these precursors into the active hormones testosterone, dihydrotestosterone, and progesterone—hormones critical to our initial and continued development as complex beings.3 The exact role of androgens in female development is cloudy, but many aspects of normal female development—including nerve and muscle cell development, as well as hormone production—do not occur normally in women with adrenal androgen deficiencies.4 Where Our Sexes Come From 57 Boys begin to produce DHEA at about age nine or ten and androstenedione a year or two later. With boys the adrenal androgens induce underarm hair and some pubic hair, but do not appear to be involved with other aspects of puberty. From birth on, the adrenal androgens act like another set of cogs in the machine that drives male sexual development. And, as we will see later, when these adrenal androgens become estrogens, which they can and sometimes do, men begin to look more like women. And th...
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This document was uploaded on 02/04/2014.

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